Giant kokopu (Galaxias argenteus)
|Also known as:||Maori trout, native trout|
|Size||Length: 20 - 40 cm (2)|
Classified as Vulnerable (VU) on the IUCN Red List (1).
Reminiscent of stars in a night sky, the gold spots patterning the dark-olive body of the giant kokupu inspired an 18th century biologist to give this fish the scientific name Galaxias (2) (3). This distinctive fish is the largest species within the Galaxiidae, a family mostly comprised of small, tubular fish with blunt heads, smooth leathery skin and a single dorsal fin, set well back towards the tail over the anal fin (3). However, the giant kokopu’s large size, stocky build, striking colouration and gaping mouth readily distinguish it from other Galaxids. In remarkable contrast with the vivid colouration and bulky body of the adult kokopu, juveniles are slim and almost transparent (2).
The giant kokupu is widely distributed throughout coastal areas of New Zealand (2).
Although some populations occur in landlocked lakes and ponds, the giant kokopu is most often found in coastal habitats, such as estuaries, lagoons and swamps, as well as gently flowing streams and rivers (2).
The giant kokopu normally migrates between the sea and freshwater as part of its lifecycle. It is still not known exactly where spawning takes place, but adult males have been observed migrating downstream in large numbers at the end of autumn (2). The females produce thousands of tiny eggs which upon hatching are carried out to sea (4). In the spring, the developing juveniles migrate back into the rivers in mixed-species shoals as part of the whitebait run (2) (4). Unlike some whitebait species, the giant kokopu does not migrate far inland, but instead remains in the slower-moving waters downstream, favouring areas where it can hide below overhanging vegetation and undercut banks (4). Maturity is reached after around three years, but giant kokopu may live for many years, with some individuals estimated to reach almost 30 years old (2).
An opportunistic feeder, the giant kokopu appears to take a wide range of prey items. In some parts of its range, terrestrial insects have been recorded as forming the bulk of its diet, while in other areas, aquatic prey are much more important. Landlocked adults appear to be particularly indiscriminate when it comes to food, even taking juvenile giant kokopu (2).
Although early colonial settlers in New Zealand documented the ready availability of giant kokopu as a source of food, today this species is encountered much more sporadically. A number of factors have contributed to its decline, but the principal cause has been the widespread loss of suitable habitat, arising from the varied impacts of agricultural and urban development. In particular, much of the aquatic habitat favoured by the giant kokopu has been severely degraded through deforestation, pollution, water extraction and eutrophication. Despite giant kokopu forming a relatively minor component of the whitebait harvest, this fishery has also been attributed to the species decline. However, since a ban was imposed in the mid-1990s on the whitebait fishery in western parts of New Zealand, data has shown that giant kokopu continue migrating for some time after the whitebait fishery normally closes. The implication of this being that capture of this species during the fishing season is probably of little concern. Other factors under consideration include the negative impact of giant kokopu caught as bycatch by the commercial eel fishery and the predatory and competitive impacts of introduced species (2).
The current focus of conservation efforts is to identify, manage and protect habitat suitable for giant kokopu. This includes purchasing land, ensuring planning policy prohibits negative impacts on suitable habitat, and raising public awareness of the threats to this species. At the same time research is ongoing to evaluate the magnitude of the various impacts on this species and to establish the most appropriate means of managing them (5).
For further information on the conservation of the giant kokopu see:
- New Zealand large galaxiid recovery plan, 2003–13:
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- Anal fin: in fish, an unpaired fin on the under surface of a fish, behind the anus.
- Bycatch: in the fishing industry, the part of the catch made up of non-target species.
- Dorsal fin: the unpaired fin found on the back of the body of fish, or the raised structure on the back of most cetaceans.
- Endemic: a species or taxonomic group that is only found in one particular country or geographic area.
- Eutrophication: excessive growth of aquatic plants that occurs when dissolved nutrients such as phosphorus and nitrogen run-off into lakes and ponds, which also ultimately increases the plant death rate with the result that the bacterial decomposition of the dead plants uses up oxygen. Natural eutrophication may occur gradually, but is often accelerated by run-off of agricultural fertilizers.
- Spawning: the production or depositing of large quantities of eggs in water.
- Whitebait: in New Zealand whitebait refers to the juveniles of six species of fish that congregate in the sea and migrate in shoals up rivers where they mature into adults.
IUCN Red List (December, 2009)